Rotors or stators of a turbomolecular pump

ABSTRACT

The invention relates to rotors or stators of a turbomolecular pump with rotor blades made of a specific aluminum alloy.

The invention relates to rotors or stators of a turbomolecular pump withrotor blades made of a specific aluminum alloy.

For the construction of rotors of a turbomolecular pump with rotorblades, aluminum has become established as a construction material,because this is the best way to combine the demand for as low a densityas possible and a high strength and ready processability. Thus, from M.Wutz et al., Theorie und Praxis der Vakuumtechnik, 2nd Ed., 1982,Friedr. Vieweg & Sohn, Braunschweig/Wiesbaden, pages 207/208, it hasalready been known to prepare the rotor or stator from especiallyselected aluminum alloys.

To be able to construct high performance pump rotors or stators,aluminum alloys having a high high-temperature strength arepredominantly employed. Rotors made of such materials are usuallyproduced by means of machining methods as described, for example, in DE10210404 A1 or DE 29715035 U1, both included herein by reference. Inparticular, the shaping of the blade contour is time- andcost-intensive.

DE 101 03 230 A1 describes rotors in which part of the rotor blades hasa back side having a convex design on the suction side and a concavedesign on the pressure side, or that at least part of the rotor bladeshas a front side having a concave design on the suction side and aconvex design on the pressure side.

When high strength alloys are used, their low reshapability is to beconsidered. As a result, the complete shaping of solid bodies or disksmust be effected by means of machining methods, and less expensiveshaping methods by plastic reshaping, such as twisting, embossing orcoining, cannot be employed.

Concretely, for the medium strength alloys, a combination of machining(turning, cutting) and/or thermal ablation methods (eroding) withmethods of plastic reshaping (twisting) has become established as a moreeconomical preparation method for rotors or stators.

In this method, single disk-shaped segments are processed at first intoa cylindrical solid body by machining and subsequently provided with anaxial slot by spark erosion. This generates disk-shaped structures perdisk segment that obtain a defined angle of attack by subsequent plastictwisting around the longitudinal axis of the blade.

DE 100 53 664 A1 describes a mechanical kinetic vacuum pump with a rotorconsisting of an Al alloy; for increasing the high-temperature strengthand creep strength, it is proposed that the rotor material be a lightmetal alloy prepared by powder metallurgy whose main alloy component,apart from aluminum, is copper, further containing magnesium, manganese,zirconium and silver and optionally titanium.

By using a novel high strength and high high-temperature strengthaluminum wrought alloy that has an unusually high elongation at break ina naturally aged state, it is now possible to employ the above mentionedless expensive shaping methods that had previously been reserved forlow-strength to medium-strength Al alloys.

WO 2004/003244 A1 describes an Al—Cu—Mg—Mn alloy for the preparation ofsemifinished goods having high static and dynamic strength properties.Surprisingly, it could be found that the alloys described herein on theone hand have a particularly high high-temperature strength and on theother have so high a ductility in a naturally aged state that a low-costrotor production by machining or thermal abrasion methods and reshaping(for example, twisting or bending) is possible.

Therefore, in a first embodiment, the invention relates to rotors orstators of a turbomolecular pump with rotor blades made of an aluminumalloy, characterized in that said alloy comprises an Al—Cu—Mg—Mn wroughtalloy.

Thus, multistage, integral rotors or stators as well as rotors orstators assembled from individual stage segments are obtainable by meansof the present invention. The rotors or stators have a low density andat the same time a high strength and ready processability.

It is particularly preferred according to the present invention for thewrought alloy employed to have the following composition:

from 0.3 to 0.7% by weight of silicon (Si);up to 0.15% by weight of iron (Fe);from 3.5 to 4.5% by weight of copper (Cu);from 0.1 to 0.5% by weight of manganese (Mn);from 0.3 to 0.8% by weight of magnesium (Mg);from 0.05 to 0.15% by weight of titanium (Ti);from 0.1 to 0.25% by weight of zirconium (Zr);from 0.3 to 0.7% by weight of silver (Ag);up to 0.05% by weight of others individually;up to 0.15% by weight of others in total; andaluminum (Al) as the % by weight balance.

As compared to other previously known alloys, the wrought alloysemployed have a higher static and dynamic high-temperature strength andan improved creep strength while the mechanical properties at break arevery good, and they are therefore particularly suitable for the rotorsor stators of turbomolecular pumps according to the invention. Inparticular, the wrought alloy employed according to the invention has anelongation at break of at least 14%, especially from 17 to 20%, in anaturally aged state as determined in a tensile test according to DIN EN10002.

The term “wrought alloy” within the meaning of the invention refers to aspecial treatment of the alloy employed according to the invention inwhich the cast structure is converted and “wrought” at an elevatedtemperature by, for example, extrusion, rolling or forging. The lightmetal is rendered more ductile thereby. Therefore, wrought alloys alsoallow further cold shaping operations, such as rolling, drawing or evenforging (for example, cold forging).

From WO 2004/003244 A1, it is known per se that these properties areachieved, in particular, at a copper-to-magnesium ratio of from 5 to9.5, especially at a ratio of from 6.3 to 9.3.

The copper content is preferably within a range of from 3.8 to 4.2% byweight, and the magnesium content is within a range of from 0.45 to 0.6%by weight. The copper content is clearly above the maximum solubilityfor copper in the presence of the claimed magnesium content. As aconsequence, the proportion of insoluble copper-containing phases isvery low, also in view of the remaining alloy and accompanying elements.This causes an improvement of the dynamic properties and the fracturetoughness of the rotors prepared from such an alloy.

In contrast to aluminum alloys further known in the prior art, thesilver content of the claimed wrought alloy is rather high with contentsof from 0.3 to 0.7% by weight, preferably from 0.45 to 0.6% by weight.Due to the interaction with silicon (from 0.3 to 0.7% by weight,preferably from 0.4 to 0.6% by weight), a curing occurs through the samemechanisms as in silver-free Al—Cu—Mg alloys. However, for lower siliconcontents, the course of the segregation is different because of thesilver addition. Although the rotors prepared from such an alloy havegood high-temperature strengths and creep strengths under coolerconditions, they do not meet the desired demands. Only silicon contentsfrom 0.3% by weight suppress the otherwise typical change of thesegregation behavior of Al—Cu—Mg—Ag alloys, so that higher strengthvalues can be achieved for the Cu and Mg contents without a loss inhigh-temperature strengths and creep strengths.

The manganese content of the alloy employed is from 0.1 to 0.5% byweight, preferably from 0.2 to 0.4% by weight. For alloys with highermanganese contents, undesirable segregation processes resulting in areduction in strength were found in a long-term high temperatureexposure test. For this reason, the manganese content is limited to 0.5%by weight. In principle, however, manganese is an alloy component thatis necessary for structural control.

For counterbalancing the reduced vacancies of manganese in view ofstructural control, the alloy contains zirconium in a proportion of from0.10 to 0.25% by weight, especially from 0.14 to 0.2% by weight. Thesegregating zirconium aluminides are usually even more finely dispersedthan manganese aluminides. Moreover, it has been found that thezirconium aluminides contribute to the thermal stability of the alloy.

For achieving a finer grain structure, from 0.05 to 0.15% by weight,preferably from 0.10 to 0.15% by weight, of titanium is added to thealloy. Conveniently, the titanium is added to the alloy in the form ofan Al-5Ti-1B master alloy, whereby the alloy automatically containsboron. Finely dispersed insoluble titanium diborides are formedtherefrom. These contribute to the thermal stability of the alloy. Thealloy may contain a maximum of 0.15% of iron, preferably 0.10% of iron,as an unavoidable impurity.

The rotors or stators of a turbomolecular pump according to theinvention having rotor blades made of the above defined aluminum alloycan be prepared, for example, by producing the rotor blades by radialseparation from individual disks or solid bodies, followed by generatinga desired angle of attack by reshaping (for example, twisting, bending,embossing, forging etc.). The preparation of the desired angle of attackoptionally includes the preparation of a defined blade contour.Alternatively, the steps of separating and reshaping may also beperformed in one operation, for example, by coining.

The operations of this process are known per se, but have been limitedto low- and medium-strength aluminum alloys, because these are the onlyones to have the required reshapability. By means of the presentinvention, however, this process can also be applied to the definedhigh-strength aluminum alloys.

Usually, the reshaping starts from disk-shaped blade stages in whichindividual blade segments are previously produced by radial separation.Separation methods within the meaning of the present invention includecutting methods, such as laser or water jet cutting, as well as eroding,machining, embossing or coining.

The combination of shaping by machining with methods of plasticreshaping, the production costs of the rotors or stators can be reduced.

EXAMPLES Example 1 Preparation of the Blades' Angle of Attack in PumpRotors of a TMP by Reshaping

From a cylindrical solid body made of the alloy AA 2016 (see WO2004/003244), disk-shaped segments concentrically superposed wereprepared by machining in accordance with the desired number of pumpstages. This produced a rotationally symmetric body consisting ofsuperposed disk-shaped ribs interconnected in the hub region. The ribthickness corresponded to the later blade thickness. Now, every rib diskwas slotted in axial direction to near the hub at regular intervals overthe circumference to form individual blade segments.

During this, the material was in a “solution-annealed, quenched andnaturally aged” state. In this state, it had a high reshapability.

Now, in the same state, the blade segments were twisted around theirlongitudinal axis. The twisting was effected with a fork-shaped grip armthat embraced a corresponding blade segment to near the blade foot andthen performed a twisting movement around the blade's longitudinal axisto the desired angle of attack. This caused the blade segment to suffera plastic reshaping in the zone near the blade foot. In this way,twisting angles or angles of attack of about 45° relative to thestarting position were easily achieved without observing incipientcracks in the blade foot zone.

With conventional Al alloys having a high high-temperature strength,such angles cannot be achieved.

To achieve the strength required for the later operation, artificialageing was performed to achieve the maximum strength according to stateT6.

Example 2 Coining of Stator Disks

From Al sheets made of the material mentioned in Example 1, stator disksare prepared by coining as follows:

Semicircular ring segments are punched from Al sheets in thicknesses offrom 0.5 to 1.0 mm. State of the sheets: “solution-annealed, quenchedand aged naturally”.

These ring segments are now inserted in a coining die that will carvethe blade contour by pressing the bottom die onto this coining die.

The leading edges of the radial-symmetrically arranged blade segmentsare punched out thereby whereas the angle of attack of the blades isproduced by plastic reshaping within the coining die. The maximumreshaping occurs in the zone of transition from the free leading edge ofthe blade to the non-deformed sheet.

To achieve the strength required for the later operation, artificialageing was performed to the maximum strength according to state T6.

1. Rotors or stators of a turbomolecular pump with rotor blades made ofan aluminum alloy, characterized in that said alloy comprises anAl—Cu—Mg—Mn wrought alloy.
 2. The rotors or stators according to claim1, characterized by comprising a wrought alloy having the followingcomposition: from 0.3 to 0.7% by weight of silicon (Si); up to 0.15% byweight of iron (Fe); from 3.5 to 4.5% by weight of copper (Cu); from 0.1to 0.5% by weight of manganese (Mn); from 0.3 to 0.8% by weight ofmagnesium (Mg); from 0.05 to 0.15% by weight of titanium (Ti); from 0.1to 0.25% by weight of zirconium (Zr); from 0.3 to 0.7% by weight ofsilver (Ag); up to 0.05% by weight of others individually; up to 0.15%by weight of others in total; and aluminum (Al) as the % by weightbalance.